I am having a unique problem with my AC and am running into dead ends. I recently had the AC compressor and everything else that goes along with that replaced because that was broken, so the car blows nice cold AC but on longer drives air will stop blowing out of the vents. I have replaced the AC blower motor and resistor and still experience the same issue.

On longer drives, around 40+ minutes I will turn the AC down both temperature wise and speed wise because the inside of the car cools down. Eventually I go to turn it back up because it starts to get warm but the air will not blow out of the vents. It sort of seeps out and it's still cold, everything sounds normal, when I go to maximum AC it sounds all good and if I turn from max AC to normal, floor, etc... I can hear what sounds like the vent door/blend door opening/closing but still no air out of the vents.

If I turn the AC off while driving for an extended period of time it will start to work again or if I turn the car off then back on it will work again. I started to play with it and noticed on maximum AC it will slowly start to reduce the amount of pressure it is blowing as the drive goes on.

I really don't know what the issue could be seeing as to how the blower motor, resistor, and compressor are brand new. FYI this issue was present before any of the new parts were installed.

Hopefully someone here has dealt with something similar and can point me in the right direction.

I was thinking it might be the blend door actuator but if that was the issue wouldn't that mess with the temperature of the air coming out, not the pressure? I might as well go ahead and swap it out, I haven't had any luck online finding where it is located in the car and mixed results on the part itself.

The links are good pics of a blend door actuator, there are many interferences that need to be removed in order to get good access to the actuator, best to purchase a FSM field service manual for your year of stang, they cost about 100 dollars but can always be resold when your need for it no longer exists. Wishing you good luck with your project

Could the issue be related to a vacuum line or the vacuum itself? I will have to check and see if air is coming out of the defrost vents next time the pressure goes out. If it was a line I would think the issue would happen all the time and it wouldn't arise after 40+ minutes of driving but if it's the part which creates the vacuum itself going out that might need to be replaced.